You're enclosing each IF condition within a set of brackets. The following doesn't give you an error, although I'm not sure it returns the result you were expecting...
=IF(A6+B6<=50,(A6*0.11)*0.2,IF(A6+B6<=1000,(A6-50)*0.06+5.5,IF(A6+B6>1000.01,(A6-1000)*0.02+62.5)))